The present invention relates to plasmids for the production of transgenic plants that are modified in habit and yield.
The growth, development and yield of a productive plant or an ornamental plant depend on the energy that the plant obtains by fixing CO.sub.2 in carbohydrates during photosynthesis. The primary locations for photosynthesis are the leaf and, to a lesser extent, the stem tissue, whereas other plant organs, such as roots, seeds or tubers, make no substantial contribution to the formation of photoassimilates but, on the contrary, are dependent as regards their growth on being supplied by photo-synthetically active organs. That means that there is a flow of photosynthetically obtained energy from photosynthetically active tissues to photosynthetically inactive parts of a plant.
The photosynthetically active tissues are referred to as sources. They are defined as net exporters of fixed carbon dioxide. The photosynthetically inactive parts of a plant are referred to as "sinks". They are defined as net importers of photosynthetically fixed carbon dioxide.
It is to be assumed that both the efficient use of products of photosynthesis and their distribution within a plant have a strong influence on the plant in several respects. The habit of plants may be mentioned as an example. Newly developing organs such as very young leaves, or other regions such as the root and seeds, are completely dependent on the photosynthetic capacity of the sources. That means that the development of such organs is dependent on the distribution inside the plant of the photoassimilates formed in the sources. The possibility of forming young leaves, or of forming roots, could have drastic effects on the habit of a plant, such as, for example, the size of a plant, the internodal distance, the size and shape of a leaf, the appearance of a leaf and the amount and shape of the root formed. Furthermore, the distribution of photoassimilates is assumed to be of very crucial importance for the yield of a plant. Thus, the total photosynthetic capacity of wheat has not changed substantially in the last few decades, whereas the harvestable yield of wheat plants for humans has increased. This is largely attributable to the fact that the ratio between competing sinks has been changed to the effect that the sinks that are important for the yield, such as seeds, take up considerably more photoassimilate than do other regions of the plant that are not important for the yield, such as, for example, the stalk. It was thus possible by, in this case, shortening the haulm to obtain in wheat a sink to source ratio that is much more favourable for humans. That underlines the importance in higher plants, in relation to both the habit and the yield of plants, of the distribution of photoassimilates formed in the primary sources.
Modifications in habit, resistance to dryness and/or frost and especially modification in the yield of plants are considerable improvements over known plants.
Biotechnological processes for the genetic modification of dicotyledonous and monocotyledonous plants are known (Gasser and Fraley, 1989, Science 244, 1293-1299).
It is not known what biochemical mechanisms regulate the ratio of sink to source.